(1) Field of the Invention
This invention relates to an improved process for calcining gypsum rock to hemihydrate. More particularly this invention relates to an improvement in the calcination of lump gypsum rock to alpha hemihydrate. In such processes gypsum rock is calcined to alpha hemihydrate under steam pressure using coarsely pulverized rock in excess of 1/2 inch (1.3 cm) diameter pieces.
Alpha hemihydrate calcium sulfate is generally prepared by dehydration of gypsum in water at temperatures above 95.degree. C. and by disassociation in an atmosphere of saturated steam. Typically in the calcination of gypsum rock, rock from the mine or quarry is crushed and sized to meet the requirements of the calcination processing e.g, either the so-called "lump" gypsum rock processes in which the rock is coarsely ground or the so-called "slurry" type of process in which a powder of the gypsum is suspended in a solution of crystal habit modifier and autoclaved.
"Lump" process gets its name from the first commercially successful means of obtaining alpha hemihydrate, having a low normal pouring consistency, disclosed in U.S. Pat. No. 1,901,051. This method involves charging lumps of gypsum to an autoclave and calcining under saturated steam pressure of 15-20 psi gauge for 5-7 hours, then drying and grinding the product. In practice small lumps are preferred and long calcination cycles are required even to produce a 40-50 cc consistency product. U.S. Pat. No. 2,907,667 taught that calcination time could be markedly decreased without substantial increase in consistency of the product if the lumps, sized between 1/2 to 11/2 inches, were first soaked in a crystal-habit-modifier solution of about 10% concentration with a wetting agent to wet them and the solution drained off; then calcining at the usual lower pressure for a length of time and then progressively increasing the pressure. U.S. Pat. No. 2,907,668 added the variant of percolating the crystal-habit-modifier solution through the gypsum rock mass, at a rate of 1 pound of 1-3% solution per 7 pounds of rock, for 3-7 hours during the early calcination, generally 3 hours, followed by 1-3 hours of calcination without the solution. U.S. Pat. No. 2,616,789 disclosed additional crystal-habit-modifiers; and U.S. Pat. No. 2,913,308 found that lower consistency product could be obtained by using smaller particles with a top size of 1/2 inch with a crystal-habit-modifier solution during the usual low pressure first part of the calcination cycle, withdrawing the solution from the partly converted gypsum and continuing the calcination under increased pressures to complete the conversion. Finally, U.S. Pat. No. 3,081,152 teaches that the processing times may be considerably shortened in the foregoing processes by a defined scheduling of incremented pressure increases during calcining. Thereby the normal calcination time of the '051 patent was reduced from about 5 hours to less than 3; the '668 percolation method from 8 to about 5 hours; and the '667 soak method reduced about 50% from the original 9-16 hours.
However there are a few gypsum rocks which are difficult to calcine into low consistency products, and further the uniformity of piece size in the crushed rock stream being diverted to lump process calcination greatly affects production.
One of these difficulties occurs when the natural gypsum massive rock includes veins of a pure gypsum crystalline form called selenite. Selenite is a tabular monoclinic crystalline form. Although the individual grains in a massive gypsum rock may actually be small, inhibited selenite crystallites, there are some gypsum ores that contain uninhibited, much larger and well formed, crystals of selenite possibly 1/4 inch to as large as 2 inches in diameter after sizing in normal crushing equipment. Being monolithic these pieces of selenite require more time at the same pressure or greater pressure in the same time for conversion than the small grains of massive gypsum require. Heat can penetrate around and into the grains of massive gypsum more readily than through the crystalline structure of selenite. For example if a selenite vein is encountered, the low consistency product may be maintained by raising the pressure e.g., to 20 psig instead of 14 psig saturated steam and/or increasing the cycle time e.g., from a 5 hour cycle to an 8-10 hour cycle in the '152 patent's cycle improvement to the Randell and Dailey '051 patent process. Thus the introduction of some selenite in the rock supply will either require wasting of calcination time and steam supply in the overcooking of the massive grains or in carrying through some unconverted product raising consistencies of the production batch. Further the resulting alpha hemihydrate continues to give more trouble in that alpha hemihydrate from selenitic crystal yields a ground product of a higher aspect ratio. Under the same conditions of grinding, alpha hemihydrate from selenite will have an aspect ratio around 2-3:1 whereas alpha hemihydrate from the grains of massive gypsum rock will have an aspect ratio much closer to 1:1. It is harder to grind, and it tends during grinding to maintain the preferred orientation for tabular form.
Another problem encountered in all lump alpha hemihydrate production is that only a small part of the rock stream from the crusher may be utilized. As larger pieces on the order of 2 inches-21/4 inches, or a higher proportion of pieces that significantly deviate from the median in a 1/4 to 11/2 inch range find their way into the rock stream, increased pressures and lengthened calcination times and/or increased crystal-habit-modifier solution must be reverted to in order to obtain uniform product. Again this results in longer cycle times, more crystal-habit-modifier solution usage and higher consistencies in the resulting product.
For both of the above reasons there is a need to improve the lump processes to maintain a low consistency product without increasing the cycle time thereby slowing the rate of production, and without increasing the steam pressure utilized thereby increasing the cost of production and encountering unreactive product in general; and further in particular being able to produce a low consistency product from rock containing substantial proportions of selenite.
Further it would be desirable while maintaining low consistency product to be able to increase the portion of rock feed coming from the rock crusher that may be utilized in the production of alpha hemihydrate; to decrease the cycle time used for massive gypsum rock and to avoid having to increase substantially both the cycle times and pressures required when veins of selenite are encountered.